Thin film stress is regarded as a quite important factor in the thin film deposition. The thin film would be possibly peeled off from the substrate, crack or wrinkle may occur while the stress existing therein is relatively large. In addition, damaged optical film may introduce unexpected scattering beams into optical system, which may lead to the production of ghost image. The optical system may thus suffer performance regression and even fail in operation. Moreover, the performance of the deposited thin film in an electro-optics device under a critical operating condition shall be also taken into consider. Generally, optical elements of a relatively high power are always adopted in a typical liquid crystal projector for achieving the high illumination, which brings the optical engine to be operated under a high temperature, so that the optical thin film or the optical coating would be easily peeled off or damaged due to the high temperature thereof. In this case, the liquid crystal projector may be disadvantageous in conducting a reduced illumination and a non-uniform lightness, and the ghost image produced owing to the damaged optical thin film therein also decreases the performance thereof. Therefore, it becomes a critical issue in this field to evaluate the ability in estimating the stress introduced by deposited thin film thereof.
Thin film stress is introduced during the coating process owing to the difference between the respective phase states of the substrate and the deposited thin film thereon. In some coating or deposition processes, such as the thermal oxidation deposition, sputtering, the evaporation and the chemical vapor deposition (CVD), it needs to heat the substrate as well as the thin film deposited thereon, which leads to the residual stress existing in the thin film.
Typically, the thin film stress is evaluated by means of non-destructive testing including the mechanical method, the interference method and X-ray diffraction method. The X-ray diffraction method is performed for a crystalline film, where the average stress of a thin film is derived from the comparison between the lattice distance of the fabricated film and the theoretical lattice constant of an undeformed lattice. Such a method, however, is not suitable for an amorphous film. The mechanical method as well as the interference method were based on the theory developed by Stoney in 1909, where the measurement of curvature radius of a deformed foil substrate in combination of the relationship between the film stress and the curvature radius of a substrate-film hybrid structure are adopted for the basis of stress measurement for the electroplated film, so as to evaluate the stress existing therein. Such methods, as well as the modification thereof, are typically adopted for the thin film stress evaluation, in particular for the stress evaluation for an amorphous film, in both the academia and the industrial circles at present days.
In order to evaluate the thin film stress precisely, it needs to increase the precision of measurement for the respective curvature radius of the substrate and the hybrid structure as possible. For this purpose, it always adopts the optical interference method or the optical leverage technique with an image processing system or sensing elements for the curvature radius measurement.
Take for examples, the optical leverage technique in combination of the image processing is developed for improving the precision of measurement for the deformation at the terminal of a cantilever structure. In 2000, Twyman-Green interferometer combining with the phase shift concept is developed for increasing the precision of measurement for the surface topography and for obtaining a whole field deformation as well. Such a method, however, still adopts an average curvature of the substrate-film hybrid structure for evaluating the average stress existing in the thin film. Moreover, the shadow moiré method combing with the three-step phase shifting method is further developed for measuring the deformation of a wafer thin film, where the average curvature of the substrate-film hybrid structure is also adopted for evaluating the average stress existing in the thin film, so as to increase the precision of measurement for the curvature radius.
Nevertheless, the mentioned methods adopt the phase information regarding the whole field only for precisely evaluating the average stress of the film, which still fail to provide a method for a whole field stress evaluation for the thin film. Furthermore, the region where the film is peeled off or cracks is initialized always a region of an local extreme high stress, rather than the region of an average stress, and thus the mentioned methods are not suitable for the crack or peel-off location prediction and/or evaluation for thin film.
For overcoming the mentioned drawbacks of the prior art, a novel method for whole field thin film stress evaluation is provided in the present invention, whereby the whole filed thin film stress distribution for an optical thin film would be developed with a commercial interferometer, so that a whole field evaluation for the crack or peel-off of thin film is hence achievable.